Badger
TGN Super Badger in a handle of your choice. I prefer a 21mm Fan, but any of their Super Badger or Silvertip knots are good. I just cannot see spending the money on big name badgers when you can get a great knot for around $25.

I don't know that it's even possible to make a list of brushes that are necessary to have. But in terms of badger brushes that I think everyone should try at least once (and I can't speak to boar or horse because I have never used them), I would have to list a Simpson Duke and a Chubby, a Manchurian, a vintage Plisson, a Shavemac silvertip, and a Morris & Forndran blonde.